Devices of the above type are generally known, and are discussed, for example, in U.S. Pat. No. 3,978,457. This patent discloses a system for a postal meter including a keyboard for the manual introduction of data corresponding to the postage to be printed.
In prior devices of the general category including electronic postage meter operation, it has been found desirable to employ one or more microprocessors to control various meter functions and operations. For security reasons, all data relating to accounting may be maintained separately from other data relating to nonsecure information. By separate maintenance of secure data it is therefore possible to improve security while employing concepts of distributed processing by the use of multiple processors.
The use of multiple processing in a secure environment places stringent requirements on error rate in interprocessor communications, which requirements must be satisfied in order to have successful operation. It is further desired to provide a system of this type which can easily subrogate its control authority to an external unit, thereby allowing substantially one hundred percent control of meter function to be transferred to an external device, without any change in system hardware. In addition, the use of electronics in postal meters allows the capability of greater sophistication in automatic recharging the accounting registers without the need for operating personnel. It is a desirable feature of any meter therefore to provide improved methods of detecting tampering and performing self-diagnostic error checking.